The present invention relates to a highly intensive cooling process and an apparatus for the production of biaxially oriented films from high- and medium-molecular-weight thermoplastics.
Conventional processes for production of biaxially oriented films from medium- and high-molecular-weight HEPE and LDPE polymers and LLDPE mixtures with the so-called "long-neck" procedure are old in the art. The "long-neck procedure" leads to improvement of the mechanical and physical film properties, enhancement of the drawdown of the polymers and achievement of better optical properties with LDPE and LLDPE polymer types.
In what is known as the neck zone of devices of known construction, the tubular film is blown up to the desired diameter. Orientation of the film occurs in the transition zone from the neck to the tubular film.
Furthermore, tests have been carried out in order to achieve higher extrusion outputs with better bubble stabilities, during which the specific die output can be increased only by intensifying the external cooling of the tubular film, i.e., by increasing the air velocity on the external surfaces of the neck and of the transition zone from the neck to the tubular film.
However, the maximum air velocity and thus the maximum cooling effect are very rapidly limited by the stability of the bubble. If the cooling air streams are too large, the instability of the neck will cause considerable upward and downward movements of the bubble, leading to wide fluctuations and wrinkling of the film and ultimately to a break-off of the tubular film. The output limit in the conventional "long neck" process lies between 1.5 and 2.3 kg/cm depending on blow-up ratio, film thickness, type of polymer, and room temperature.
Japanese Pats. No. 55/2180, 53/133,275 and 59/136,224 disclose processes by which the bubble stability was improved and the specific die output increased by the introduction of an internal drag mandrel in the neck zone. However, here the output limit is raised only 20% compared to the conventional process, approximately 1.8 to 2.75 kg/cm.
Further output gains still with good bubble stabilities are achieved by the combination of an external cooling immediately after the melt's emergence from the die, a stabilization with an internal drag mandrel, and a second external cooling on the bubble or in the transition zone.
Such processes are described in Japanese Pat. No. 59/171,620 and in U.S. Pat. No. 4,626,397. These processes, which use a second cooling annulus, are not quite operator-friendly, especially upon starting up the system and when converting the program. The second cooling annulus in the drag mandrel area gives rise to operating problems.
West German Pats. No. 2,262,190, 2,306,831, 2,610,818 and 2,459,785, teach processes which are used for high-speed extrusion of LDPE and LLDPE with the conventional bubble shape or with a short neck. Because of the instability of the bubble, the process with the so-called "long neck", with a height of 6 to 10 times the die diameter, is possible only with restrictions.
An improvement of bubble stability was achieved with U.S. Pat. No. 4,606,879 and with West German Utility Model 85 25 622.6. But here, too, neither system is particularly operator-friendly, because of the second cooling annulus or because of the chambers with iris dampers. Also, the conversions of the production program are sensitive in both systems. A further disadvantage, especially in West German Utility Model 85 25 622.6, is the condensation of the monomers and of the additives to the polymers on the inner surfaces of the chambers and on the iris dampers. Frequent cleaning of the apparatus is necessary in order to ensure the operation of the system.
Therefore, the present invention has as its object the provision of a highly intensive cooling apparatus for the production of biaxially oriented films from high-and medium-molecular-weight thermoplastics, which overcomes the disadvantages described above and features at the same time a combination of an intensive external cooling, which is effective immediately after the melt's emergence from the mold, a stabilizing mandrel positioned in the area of the neck immediately ahead of the expanding area of the bubble, and a highly intensive internal cooling system which is activated immediately after this drag mandrel in the expanding area of the bubble.
This new apparatus results in a significant increase in the specific extrusion output of the die, substantial improvement of the mechanical and physical properties of the film, very close thickness tolerances, and the perfect bubble stability, the exact maintenance of the film width, and the absolute freedom from wrinkles of the lay-flat tubing are always ensured. Furthermore, the present invention is to provide an extremely operator-friendly apparatus, which is particularly flexible and can be used for a very wide range of applications, and which also permits in but a few minutes the startup of the system or a conversion of production.
The invention achieves this object by leading the neck at the expansion zone: in two areas, on a conical contact surface of the drag mandrel and on point contact of air lips, both of which are inside the tube neck, and by activating the internal cooling system immediately after the point contact in the expansion zone of the bubble; or in three areas: on the conical contact surface, the point contact, and on a contact surface of guide rings, on the long-neck and by activating the internal cooling system immediately after the point contact in the expansion area of the bubble, by passing internal cooling air through a coaxial tube extending through the drag mandrel, to the air lips, and sucking the hot air from the internal cooling system centrally through the drag mandrel via another of the coaxial tubes.
Furthermore, an apparatus for carrying out the method of the type mentioned in the introduction is characterized by the fact that it has a first tube mounted on a die, with an internal, second tube disposed therein and inserted into the die and in communication with an exhaust conduit, that it further has a conical drag mandrel disposed around the first external tube, and a terminal exhaust tube provided with air lips, that there is disposed in an end zone of the second tube a centering means that locates the second tube, there being formed in a main manifold at least one cooling channel which leads to a cooling channel that is formed by the first tube and by the second tube mounted on the air main manifold, that the lips of the have at least one cooling channel, that the outside diameters of the air lips are in congruent relationship with the outside diameter of the conical drag mandrel, or that the air lips have a portion that projects beyond the drag mandrel and forms a point contact, that channels that operate for pressure equalization are formed between the die and the main manifold, that there is formed between the drag mandrel and the first tube a gap which is in communication with vent bores disposed in the tube, as well as slots that operate for pressure equalization and are disposed near a centering means between the drag mandrel and the first tube, that a thermal expansion-compensation device in the form of a cup spring is disposed between the air lips and the terminal exhaust tube, that the apparatus is attached with a bayonet lock to a split ring and thus ensures rapid disassembly of the device, that the drag mandrel is covered with an insulating material, preferably of cotton, the drag mandrel is covered with an insulating ceramic coating, that the conical surface of the drag mandrel has a coarse thread or a wave-like profile in order to reduce the contact area, that at least one guide ring that forms a contact surface is disposed on the first tube, that the guide ring is provided with a coarse thread in order to reduce the contact area, and that the drag mandrel has a set screw for vertical adjustment.